1,720,995 research outputs found
Regarding enumeration of molecular isomers
No full textIn this paper, a simple theoretical approach to counting of substitution isomers is described. It is based on Polya's theorem and on point groups as recently described by us [Baraldi, I.; Fiori, C.; Vanossi, D. J. Math. Chem. 1999, 25, 23-30] and extended in this paper. Several applications are developed that range from molecules without symmetry to molecules with icosahedral symmetry (Ih) The problem of the appearance of stereoisomers is also analyzed
On the character tables of finite point groups
No full textA new method for the "complete" construction of the character tables of all the finite point groups is presented. In an explicit way, only the character tables of the five nonisomorphic rotational groups (Cn, Dn, T, O, I) have been derived, because those of the remaining groups can be obtained through isomorphisms and direct products. The characters of the 1-dimensional irreducible representations are calculated using the generators and the defining relations of the groups, while for the multi-dimensional ones explicit formulas are given
Chemical asymmetry and alpha and beta polarizabilities of D-A-D' chromophores: a three-state-model and TDDFT-SOS analysis of apenta-heptamethine ketocyanine
No full textThe essential-state model, here amounting to a three-state model, has been employed to account for the effects of chemical asymmetry on the electronic alpha and beta polarizabilities of a pentaheptamethine ketocyanine (KC2,3), a prototypic D-A-D' chromophore. A suitable model, based on the idea of a ‘chromophoric site’, has been set up in terms of the three-state model features previously derived for the parent symmetric pentamethine and heptamethine ketocyanines, KC2 and KC3. This approach has been found to reproduce very well the experimental transition energies and dipoles. From the resulting properties of the ground and two relevant lowest excitedstates, average alpha and beta vec have been evaluated according to the SOS approach. The performances of themodel have been tested by comparison with the results of TDDFT SOS (hyper) polarizability calculations considering up to twenty excited states. A detailed analysis of the results for the threeketocyanines has shown a rapid convergence of the SOS expansion that supports the reliability of descriptions based on a few low lying excited states (here corresponding to p - p* excitations). However, while only two excited states were necessary for the symmetric compounds, for KC2,3 avalue of beta vec comparable with the converged value, as well as with that predicted by the experimentally-based three-state model, has been obtained including at least three excited states.Both the TDDFT SOS and the three-state model descriptions have emphasized the important role played by the three-level term contributions in the determination of beta vec. Moreover, bothdescriptions agree in predicting that KC2,3 features average alpha and beta vec values in between those of KC2 and KC
First- and Second-Order Polarizabilities of Simple Merocyanines. An Experimental and Theoretical Reassessment of the Two-Level Model
No full textTaking four merocyanines [(CH3)2N-(CHdCH)n-C(CH3)O; n ) 1-4] (Mc1-4) as test D-A systems, weperformed a close experimental and theoretical examination of the two level model with reference to itsability to provide correct predictions of both absolute values and dependence on the conjugation path lengthof first- and second-order molecular polarizabilities. By 1H NMR spectroscopy merocyanines Mc1-4 werefound to be ∼1:1 mixtures of two planar conformers with cis and trans arrangements of the sC(CH3)Oelectron-acceptor group and all trans structure of the polyene like fragment. The degree of bond lengthalternancy (BLA) in the -(CHdCH)n- fragment, was quantified by extensive full geometry optimizations atboth semiempirical and ab initio level. DFT (6-31G**/B3LYP) optimized geometries were considered to bemost reliable and were used for calculations of the excited-state properties. The applicability of the two levelmodel, reducing the general sum-over-states (SOS) expansion to only one term involving the ground state (g)and the lowest-lying 1(ππ*) CT state (e), was checked by analysis of fluorescence and near UV absorptionspectra. Measurements of the basic two-level model quantities (Ege, μge and Δμeg), by which the dominantcomponents of r and tensors are expressed (RXX, XXX, X ≡ long molecular axis), were designed to giveapproximate free-molecule values. It is proposed, in particular, an adjustment of the solvatochromic methodfor the determination of Δμeg, based on accurate measurements of absorption spectral shifts in n-hexane/diethyl ether mixtures with small diethyl ether volume fractions. Such an approach led to Mc1-4 XXX’smatching well in both value and n-dependence with EFISH data reported in the literature for similarmerocyanines. For the fluorescent Mc4, the results were qualitatively well reproduced by an approach, whichcombines absorption and fluorescence solvent shifts. All the measured quantities were calculated for bothtrans and cis Mc1-4 by three semiempirical INDO-based approaches aiming at evaluating the performancesof different integral parametrizations and CI extensions: ZINDO/S, CS INDO SCI, CS INDO SDCI. In allcases, RXX and XXX were found to rise proportionally to about n1.3 and n2, respectively, in qualitatively goodagreement with the experimental values. As to the absolute values, however, experimental RXX’s and XXX’swere best reproduced by CS INDO SDCI combining Ohno-Klopman parametrization and CI including bothsingle and double excitations. The validity of the two-level model was checked by comparison with convergedSOS calculations for the longest chain merocyanine (Mc4) and finite field calculations of linear polarizabilitiesfor all of the four dyes (Mc1-4)
On the cycle index of point groups
No full textIn this brief work we express the cycle index of the molecular point groups as a function of a limited number of initial geometrical parameters. Such parameters are the number m of elements composing the domain D of sites of substitutions in the molecule belonging to the point group G, and the numbers (n(C-n),n(sigma),...) of sites of D lying on the symmetry elements (C-n,sigma,...) for G
Rotamerism and electronic spectra of aza-derivatives of stilbene and diphenylbutadiene. A combined experimental and theoretical study
No full textThe experimental results on the rotameric equilibrium and electronic spectra of aza-derivatives of trans-stilbene and 1,4-diphenylbutadiene, have been rationalized by a theoretical study which combines simple ab initio calculations of molecular energies for the ground state with a theoretical analysis of the splitting of the conjugation band developed at CS INDO Cl level. All results indicate that the stable conformer of each ortho aza-derivative is that corresponding to A species. As suggested by the H-1-NMR experiments, the ab initio geometry of ZE-2-pyridylphenylbutadiene is consistent with the presence of the N.H intramolecular hydrogen bond. As regards the Franck-Condon excited states of aza-derivatives, our theoretical results show that the first singlet excited state has (pi(H), pi(L)(*)) character in all compounds except for E-4,4'-dipyridylethene, where S-1 has (n, pi*) character in non-polar solvent. In this last compound, the theoretical study of solvatochromism indicates a crossing between the (1)(n, pi(L)(*)) and (1)(pi(H), pi(L)(*)) states which occurs in solvents of high polarity. The inclusion of the most important. doubly- and triply-excited configurations in the CI calculations shows that the (1)A(g)(-) excited state is above the spectral region analyzed. (C) 2002 Elsevier Science B.V. All rights reserved
Linear and nonlinear optical properties of V-shaped D-π-A-π-D chromophores: Effects of the incorporation of aromatic rings in the polyenic π-bridges of open-chain ketocyanines
No full textFollowing previous studies on a and b polarizabilities of ketocyanines, a subgroup of D–p–A–p–D quadrupolar
chromophores with moderately V-shaped structure, the present work analyses the effects of modifying the
p-bridges connecting the D (NMe2) and A (CO) groups. This aim is pursued through a detailed comparison
between the previously studied ketocyanines (KC2, KC3) and a Michler’s ketone analogue (KM1) bearing
styrenic (in the place of polyenic) p-bridges. First, we report a spectroscopic study, including absorption
and fluorescence anisotropy spectra, aimed to probe the electronic peculiarities of KM1 as well as to
derive consistent three-state model (TSM) parameters for the three compounds. The paper goes on with
an extensive theoretical study, carried out in the framework of the density functional theory (DFT),
encompassing the structure, the electronic spectrum, a and b polarizabilities and two-photon absorption
(TPA) cross-sections (sTP). Calculations performed according to the sum-over-states (SOS) approach are
discussed with reference to the performances of few-state descriptions, it is shown that such
descriptions (including TSM), which have been proved to be quite reliable in the case of KC2 and KC3,
lose their effectiveness with KM1 because of the electronic characteristics related to the styrenic
p-bridges. As to the TPA cross-sections, the results of TSM and SOS approaches concerning the TSM
g - c and g - e transitions are supplemented by those obtained using the quadratic response theory.
A common qualitative conclusion, traceable to the degree of bending of the V-shaped structure, is that
in the case of KM1 the allowed (g - e) and the ‘‘forbidden’’ (g - c) transitions both should be
observable in the TPA spectrum, as confirmed by experiment
Exciton effects in the dimer and higher aggregates of a simple merocyanine dye. A CSINDO CI based theoretical study
No full textA simple streptomerocyanine with five methine carbons (Mc) has been used as a suitable model system to investigate theoretically the optical properties in organized assemblies of merocyanine dyes. The subject was treated by a methodology making use of a semiempirical quantum chemistry description of the isolated chromophore within the Frenkel exciton theory. Both an extended CS INDO CIPSI and easier CS INDO SCI (with different parametrizations of electron repulsion integrals) schemes were applied in order to establish the role played by the quality of the wave functions. The exciton states were built on the (1)(pi pi*) (HOMO-LUMO) excited-state responsible for the color band of Me. Exciton effects were first analyzed for two stable dimer structures, having H- and J-type character, located by use of a semiempirical intermolecular potential and the simulated annealing method. Then, attention was shifted from dimers to higher-order model aggregates built in such a way as to obtain typical H (columnar) and J (staircase) arrangements. Exciton-state properties (energies and transition dipole moments) were studied as a function of the aggregation number N (up to N = 50). The results are discussed with reference to the approximations used to calculate the excitonic matrix as well as to the effects of changing from CIPSI to simple SCI schemes
Electronic spectra and fluorescence properties of multichromophoric sulfonylureas
No full textThe multichromophoric character of two sulfonylurea herbicides, SMT and BNS, has been investigated in its manifestations in the electronic absorption spectra and in some fluorescence properties through a combined experimental and theoretical approach. After a theoretical analysis of the most stable structures of these flexible systems, the UV absorption spectra of the two multichromophoric compounds have been analysed both experimentally and theoretically, and most transitions have been assigned to individual chromophores, also by comparison with four suitable reference compounds (5-8). Finally, some experimental information concerning the fluorescence spectra and quantum yields have been analysed with reference to the contributions from single fluorophores and the role of low-lying n -> pi* states. (c) 2006 Elsevier B.V. All rights reserved
Solvent influence on absorption and fluorescence spectra of merocyanine dyes: a theoretical and experimental study
No full textThe solvaton-CS INDO model, previously successfully used to describe the solvatochromic properties of merocyanines, has been extended to the study of the solvent influence on the fluorescence spectra (fluorosolvatochromism) of these dyes. A ketocyanine (M1) and a stilbazolium betaine (M2) were chosen as representatives of positively and negatively solvatochromic behaviours, respectively. The gap of experimental knowledge concerning the emission properties of M2 was filled by a spectrofluorometric analysis in a set of solvents covering a large range of the E-T(30) scale. Solvato- and fluorosolvatochromism were described by calculating the S-0 (eq.) --> S-1 (Franck-Condon) and S-1 (eq.) --> S-0 (Franck-Condon) transition energies as a function of a polarity factor related to the static dielectric constant of the solvent, and ranging from 0 to 1. The absorbing S-0 (eq.) and emitting S-1 (eq.) units (solute molecule + solvent cage) were approximated using the S-0 and S-1 geometries of the unsolvated molecule and the respective charge distributions fitted to the current value of k(epsilon). The calculation results fully confirm that S-0 and S-1 states of merocyanines can be viewed as a mixture of a neutral and a zwitterionic structure whose composition is controlled by the solvent polarity. The plots of the calculated spectral data (absorption and emission maxima and corresponding Stokes shifts) vs k(epsilon) are in fairly good agreement with those of the experimental data over almost the entire range of the normalized E-T(N) values, thus showing that specific solvent interactions are at least partly simulated within the solvaton-CS INDO scheme. The methodological prerequisites for a correct prediction of solvatochromic shifts are recalled with reference to previous conflicting theoretical interpretations
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